“Snowpiercer” is its beginning. But believe me, it’s not alone. Before a surprising cinematic July is over, you’ll see another authentically great and rare sci-fi film of huge ambition and accomplishment, Mike Cahill’s diametrically opposite “Origins,” a film of entirely different intellect and emotion but equally thrilling audacity.

If you’ve been waiting for independent movies beyond the usual blockbuster hoo-ha to stir the soul, July is when it’s happening.

“Snowpiercer” is the film, in fact, some have been waiting for all year – an authentic film vision worth buttonholing people on the street about and nabbing passing friends in the hall to talk their ears off. If it isn’t, as a few have called it, the first authentically great film of 2014, it certainly leads to that discussion. This, we can all say within just a few minutes of its dark, grubby opening, is what movies are for.

The apocalyptic sci-fi premise is both plain and sturdy. The year is 2031. To combat global warming in our time, scientists invented something called CW-7, a substance to sprinkle in the upper layers of the atmosphere to reintroduce cold to the world’s climate. It’s characteristic of humankind that its solution to global warming is so effective that it freezes the entire world to a temperature where no living creature, including humans, can survive.

Except, that is, for the inhabitants of a giant bullet train running on a perpetual motion engine and rigidly stratified into socioeconomic groups from front to back.

The entire world’s surviving population, then, is in motion aboard an immense train which, as big transports so often do, makes sure that the wealthy ride in one place in maximum comfort and the deprived have to be content to merely survive in steerage. In this case, it’s the back of the train, where the majority are encrusted with filth from the absence of water and their only food comes in gelatinous, coal-black bars of protein goo whose revolting origins are discovered two-thirds of the way into the film.

Missing arms and legs are mysteriously rife among those trying to survive in the rear of the train. A common punishment for the slightest rebellious act or comment is to stick one of the transgressor’s arms through a hole in the car into the frozen atmosphere, lock it in place long enough to turn it into a frozen armsicle and then smash it into frozen cubes of flesh after it has been pulled back inside.

The details of this cruel world escalate in horror and violence and suspense – until the moment for “revolution” comes and a leader played by Chris Evans is determined to get to the engine at the front of the train or die. The brains behind him are possessed by an elderly man missing a few limbs played by John Hurt. The leader’s second-in-command revolutionary is played by Jamie Bell.

As they get farther and farther up the train – and therefore farther up into the train’s privileged classes – the 1 percent’s luxuries, when revealed in full color, seem shockingly decadent and enraging in contrast to the monochrome gray of the filth in back. The film is brilliantly designed and shot – not exactly subtle but enormously powerful. It’s a horror comic freakshow version of the distribution of wealth we’ve become used to.

In the engine proper, is the train’s inventor Wilford, who is only finally seen at the end. The actual enforcement of the train’s systematic and killing deprivation is done by thugs who seem to be led by a hideously comic harridan played by Tilda Swinton with crazily ill-fitting dentures, huge glasses, rat’s nest hair and a ridiculous way of explaining the train’s social rigidities.

Back in the rear, she assures them all, they’re like the equivalent of a shoe. At the front, they are the head. It’s the wild and wonderful performing surrealism of Swinton that is, if you ask me, the making of the film – the slice of lunatic imaginative freedom that makes the time fly by so exhilaratingly.

It’s been reported – by the Boston Globe’s Ty Burr among others – that Korean co-writer and director Bong Joon-lo ran afoul early on with Harvey Weinstein over the film’s length. Another reason, perhaps, may be that the movie’s villain with the subzero theories of social organization, is named Wilford, whose company through most of the film is symbolized by a big W which bears more than a passing resemblance to the big W logo of the Weinstein Company.

Weinstein is certainly a boss who might inspire a bit of impudent creative rudeness and chutzpah from a filmmaker and he’s also one who might find a way, however brief, to reaffirm the true place of struggling creators in the big money world of movies.

On the other hand, he’s also a mogul who might, in some deeper part he himself may not understand, sympathize with a creator shoving his chutzpah into a controlling businessman’s face.

Whatever the internecine battles, all that counts is that “Snowpiercer” is a brilliant, wildly unusual sci-fi thriller ready to be seen.

And the way I look at it, for certain kinds of people virtually demanding it.